Nanoscale AC Electroosmotic Flow and the Frequency-Size Scaling Observed beyond the Charge Relaxation Regime.

We present the first observation of AC electroosmosis (ACEO) flow at nanoscale structures in the MHz range, offering new insights into electrokinetic phenomena in this frequency regime. Our findings reveal that ACEO occurs at frequencies much higher than previously anticipated, even surpassing the Debye-Hückel frequency associated with charge relaxation. Despite remaining undetected for over two decades, this result is corroborated by the frequency-size scaling laws derived in this work from the existing ACEO theory. We contextualize these findings within the historical development of electrokinetic phenomena and discuss their implications for the current theoretical framework of ACEO.